The present invention relates to an audio amplifier and, more particularly, to a Class A complementary single-ended push-pull amplifier.
An audio amplifier is known which includes a Class A complementary single-ended push-pull amplifier circuit as an output stage power amplifier for driving a loudspeaker. In FIG. 1, there is shown an arrangement of such an audio power amplifier in which an input signal is applied to an input stage complementary push-pull amplifier circuit comprised of complementary field effect transistors Q1 and Q2. Complementary output signals of the input stage amplifier circuit are amplified by a pre-driver stage complementary push-pull amplifier circuit having complementary bipolar transistors Q3 and Q4 and then are applied to driver-stage complementary bipolar transistors Q5 and Q6. The driver transistors Q5 and Q6 are Darlington-connected to output bipolar transistors Qn and Qp, respectively. An output current Io derived from a connection point between emitter resistors R.sub.En and R.sub.Ep (of resistance R.sub.E) of the output transistors Qn and Qp is fed to a load Rl such as a loudspeaker.
A constant DC voltage source 1 for producing a constant voltage V.sub.E is connected between the bases of the driver transistors Q5 and Q6 to operate the transistors Q5 and Q6, and Qn and Qp in the Class A mode.
Let the collector currents of the transistors Qn and Qp in the above-mentioned circuit be Icn and Icp, respectively. The sum of the currents is given by EQU Icn+Icp=[V.sub.E -(V.sub.be1 +V.sub.be2 +V.sub.be3 +V.sub.be4)]/R.sub.E
where V.sub.be1, V.sub.be2, V.sub.be3 and V.sub.be4 are base-to-emitter voltages of the transistors Q5, Q6, Qn and Qp, respectively.
It is known that the base-to-emitter voltage V.sub.be -to-collector current Ic characteristics of a bipolar transistor generally exhibit a non-linear characteristics (exponential characteristics) at a rising portion of the collector current Ic. The base-to-emitter voltages V.sub.be1 and V.sub.be2 in the above-equation are set in a linear portion of the V.sub.be -to-Ic characteristics curve, because operation ranges of the transistors Q5 and Q6 are relatively narrow. Since the transistors Qn and Qp operate in a wide range, the voltages V.sub.be3 and V.sub.be4 may reach a non-linear portion at large signal levels. Therefore, odd-order harmonic distortion is produced in the output signal.
As shown in FIG. 2 illustrating the output characteristics of the amplifier of FIG. 1, the output current Io=Icn-Icp non-linearly changes as a voltage Vi between the mid point of the DC voltage source 1 and a connection point between the emitter resistors R.sub.En and R.sub.EP becomes large. Thus, for a large input signal, the odd-harmonic distortion occurs in the output signal.